The present invention relates to a semiconductor integrated circuit technology and, more particularly, to a technology which is especially effective when applied to an AB grade operational amplifier including MOSFETs, such as a technology which is effective when utilized in a transmitting buffer amplifier for driving loads of low impedance such as the subscriber's lines of the ISDN communications net (i.e., Integrated Services Digital Network).
In recent years, the transmitting buffer amplifier for driving the low-impedance loads such as subscriber's lines has been packaged in a communications LSI such as the CODEC. Since this communications LSI is required to have a low power consumption, there is frequently used an AB grade amplifier which has a push-pull operation. In the AB grade amplifier, however, the steady current, which is to be suppressed as much as possible, is liable to be fluctuated by the process dispersion, and the stabilization of the steady current is an important target.
Thus, there has been proposed an invention (as disclosed in Japanese Patent Laid-Open No. 68308/1987). According to this disclosure, some of which is shown in FIG. 12, there is interposed between a folded cascode type differential input stage 1 and a complementary type push-pull output stage 3 a level shift stage 2 for level-shifting the output of a differential amplitude stage, so that the amplifier may perform the AB grade operation having a high driving ability while suppressing the steady current by increasing the voltage to be applied to the output stage 3.
In order to realize an AB grade amplifier having a low power consumption and a high driving ability, it is a key point that the variable range of the output of the differential input stage 1 is wide. In the above-specified invention, therefore, the potential of a well region to be formed with differential MOSFETs M13 and M14 is made negative, and MOSFETs M15 and M16 used in a cascode portion are those which have their threshold voltages raised to an extent corresponding to the band gap of silicon by doping the gate electrodes with an impurity of conduction type opposed to that of the drain regions. As a result, the variable range of the output of the differential input stage 1 is increased and is level-shifted to drive output MOSFETs MO1 and MO2 to provide an amplifier of high driving ability, which can drive the loads of low impedance with the small-sized MOSFETs.